To define the role of gap-junctional communication in tumor promotion, nonpromotable, promotable, and tumorigenic transformed epidermis-derived cells of line JB6 as well as NIH 3T3, cells were subjected to the tumor promoter 12-0- tetradecanoylphorbol-13-acetate (TPA). Cell-cell communication was measured either by the radioisotope transfer technique or by microinjection of fluorescent dye. Our results give evidence for the importance of blocked cell-cell communication in focus-formation during the process of tumor promotion; however, reduced intercellular communication is not a decisive factor in maintaining malignancy. Interruption of gap-junctional intercellular communication was used as indicator in a short-term test model to uncover tumor-promoting properties in chemical agents, such as Ni-(II)-salts.